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Generator sizing doubts

Generator sizing doubts

Generator sizing doubts

(OP)
Hello,
I'm on a project where we will be using a 410 kw prime diesel generator to run three motors: a 30 Hp, 30 Hp, and 335 Hp.  All motors start across the line, and the 335 will start first.  The generator can produce 617 peak amps.  The big motor has an FLA of 400.  I have doubts that the generator will be able to start the big motor.  The generator dealer disagrees.  Does anyone have some information to convince me that the generator can produce enough current to start the big motor?

thanks

RE: Generator sizing doubts

Why not?  What does the generator sizing software say?  That big motor is about 60% of the generator capability and as long as it starts, you don't care about voltage dip.  So you will have a reduced voltage start.

I'm sure waross will weigh in with more details.

RE: Generator sizing doubts

I believe it will start it.  I also expect you to see a large voltage dip.  Do you have equipment that won't like that?  

Keith Cress
kcress - http://www.flaminsystems.com

RE: Generator sizing doubts

That will depend on the torque requirements of the motor and its load.  A motor can often start with voltage down to 50% of rated as long as it is unloaded and provided the generator can hang in there long enough.  The torque provided by the motor varies with the square of the voltage.  This will be a reduced-voltage start, even with an across-the-line starter.

I would be concerned as well.  I'd suggest a motor starting analysis.   

David Castor
www.cvoes.com

RE: Generator sizing doubts

(OP)
The vendor's generator software says it will start, but if I can't make sense of it with a calculator, then I am leery.  The motor will draw between 2000 and 2300 amps on start up, which is almost 4 times the generator's rating.  According to the vendor's project load analysis, the voltage dip will be near 30% during start up.  Nothing else will be running, so the voltage drop won't matter much.  There is the possibility that the 30% drop might drop out the motor contactor, but I doubt it.

I would like to see some documentation which says the generator can provide 400% peak current for 4 seconds.  That would make me feel better about this.    

RE: Generator sizing doubts

Why?  At 70% voltage the motor won't draw 2000-3000A.  The motor will take what it can get, but it can't get more than there is.  If what it can get is enough, then it starts, otherwise not.  If unloaded it should work if the sizing program says it will.

RE: Generator sizing doubts

What are the three motors doing? What is this thing?  That would help with deciding.  

Keith Cress
kcress - http://www.flaminsystems.com

RE: Generator sizing doubts

The last install that I did was pushing the envelope. Cat software said "NO WAY".
The customer owned an unused generator and was not about to buy another. I explained the amount of load curtailing that would have to be done and he said;
"Fine. We can live with that. Go ahead."
It turned out that although we did have to curtail loading, we could handle quite a bit more load than the software allowed.
But you are pushing the envelope as far as motor starting on a gen set.
I find that equating HP to KVA is acceptable for motors on generators. The combined efficiency and PF is often close to 80% and using .746 rather than .8 is a close approximation.
On that basis I would want to see 670 KVA or 536 KW on the gen set.
But all is not lost. Almost all diesel generators in that size range use an Automatic Voltage Regulator (AVR) which incorporates a feature known as Under Frequency Roll Off (UFRO).
When the 335 HP motor starting load hits the generator, the speed and frequency will drop quite a bit. UFRO generally allows about 3 Hz frequency drop and then starts dropping the voltage in proportion to the frequency drop. This gives a constant Volts per Hertz ratio. (With a small 3 Hz. offset).
What we have here is the poor mans VFD.  The reduced voltage avoids the high currents expected with DOL starting.
Possible issues.
Trips> Starting current will still be high but not as high as DOL on the grid. You may have to adjust the current trip settings.
Low voltage> You may have to use a UPS to provide control power during heavy starting.
Gen Set duty> If you have a prime rated set, add 10% to the generators KVA and KW ratings and use the higher ratings to predict allowable motor starting loads. For a standby rated set you must use the nameplate ratings.
Frequency> If your set has only basic controls you are probably good to go. If you have a more sophisticated control scheme you may have to disable under frequency tripping.
Bottom line> You project may well be doable, but be guided by itsmoked's comment regarding the big picture. When you push the envelope this much, you have to have all the factors. Being on site is even better.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Generator sizing doubts

(OP)
The two smaller motors are conveyors, feeding rocks into and out of a rock crusher.  The large motor is a crusher.  The crusher will never start loaded.  I don't know how long the crusher will take to start, but I am assuming 3-5 seconds max since it is unloaded.  The controls are currently set up to force the motors to start and stop in a "proper" sequence, i.e. the downstream devices must start first and stop last.  Given this condition, the 335 Hp will have to start with one 30 Hp already running.  The 30 Hp conveyor may be loaded, so this removes 40A from the generator's capacity.  If I have to, I can set up a timer which allows the crusher to start first, as long as the downstream conveyor comes on within 1 minute of the crusher starting.

However this thing starts, that generator is going to produce a big groan on start up.

 

RE: Generator sizing doubts

Try modelling the genset in the software using a reduced voltage starter.  The peak voltage dip should drop considerably.  Adding a soft starter may be more economical than increasing the size of the genset, especially if it may have some side benefits for the crusher.  

Schneider recommends a current limit of 400% FLA for their solid state soft starts in crusher applications, so try using this number in your software.  Make sure if you use a solid state starter that you include an automatic bypass to reduce harmonics on the generator.

RE: Generator sizing doubts

I'd not use a soft starter in this application. The generator and the soft starter will not play nicely together.  With the voltage sag, speed sag, and the UFRO working together the generator will be a very nice soft starter all by itself.

RE: Generator sizing doubts

I agree with David on this - the soft starter may cause more issues than it solves.  

Since the motor always starts unloaded, I suspect it will start OK.  

You do need to give some thought to the motor starter, if there is one.  The contactor can start to dropout or chatter around 85% voltage on the control power.  I suspect your voltage at the starter will be less than that initially.   

David Castor
www.cvoes.com

RE: Generator sizing doubts

david,

Any specific reasons why a soft start would cause problems?  I was under the impression that as long as it is a permanent magnet generator there should be no problems with the voltage regulator circuits.   

RE: Generator sizing doubts

I missed the note that this generator will be prime rated. Consider it as a 451 KW generator for motor starting.

Quote (davidbeach):

With the voltage sag, speed sag, and the UFRO working together the generator will be a very nice soft starter all by itself.
That's what I was trying to say. David used a lot less words.
You may consider going one step further and closing the starter for the large motor before the generator is started. You will need a source for control power to do this. A UPS for just the crusher controls should work well.

By all means, don't try to start the large motor with a 30 HP motor already running.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Generator sizing doubts

The generator (the AVR in particular) won't do well with the highly distorted load that soft starter will produce.  The only reason you can get away with them on commercial power is that they are so very small compared to the total amount of generation.

I like the idea of connecting the motor prior to starting the generator.  If the motor is unloaded and the field is applied gradually, so there is a longish voltage ramp, you could get very little "inrush".

RE: Generator sizing doubts

How are you (your customer) going to ensure the rock crusher will "always" start unloaded?  

Seems that there is a (not often but significant!) number of times that rocks will be in the crusher (after an emergency shutdown, jam, generator trip, crusher fault) and can't be hand-removed before re-starting.     

RE: Generator sizing doubts

"The generator (the AVR in particular) won't do well with the highly distorted load that soft starter will produce."  

Not trying to get off topic, but I don't agree with this as a blanket statement.  Gensets with three phase averaging AVR's (as opposed to single phase half-wave regulators) are commonly used in applications with high harmonics to prevent issues.  Also, with a bypass on the soft starter, harmonic heating of the windings would not be an issue.

I would like to see any reports to the contrary, as I will be commissioning one such system soon and would like to be prepared for potential headaches.

RE: Generator sizing doubts

Most, (all?), of the crushers that I have seen depend on the inertia of the flywheel to crush the larger rocks. A rock that can not be removed by hand will probably have to be removed by machinery of some kind before the crusher can be started.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Generator sizing doubts

(OP)
The crusher has some sort of clutch system which allows the motor to disengage from the crusher wheels.   

RE: Generator sizing doubts

I haven't had problems with soft-starters on generators as long as the generator uses a suitable regulator. Typically, an RMS measuring regulator works very well. I've seen issues with poor regulators, in one case I recall the voltage would drop almost 100V as soon as the soft-starter began to draw current.

RE: Generator sizing doubts

I haven't run into that problem (YET) Lionel. Were you able to reduce the voltage drop with a better quality AVR?  I may run into this myself someday.
Thanks
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Generator sizing doubts

Quote (LionelHutz):

as long as the generator uses a suitable regulator
No doubt, but when every last kW is being pinched out of the generator size, what are the odds of a suitable regulator?

RE: Generator sizing doubts

Hi David. With a soft start I would be concerned with the regulator possibly misinterpreting the voltage distortion due to the part cycle conduction combined with the relatively high impedance of a generator and pushing peak voltages too high.
Another issue may be a d'Arsonval based meter giving an inaccurate indication on a distorted wave form.
On the other hand, if the frequency is being dragged down I want to see a voltage drop.
But, I won't challenge Lionels experience.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Generator sizing doubts

Most VFDs and soft starters do not tolerate fluctuations in incoming frequency very well. Frequency regulation is only as good as the governor on the diesel engine.  Unless the engine  is substantially oversized to the load then anticipate problems with the VFD or soft start faulting. When the diesel engine is oversized enough to maintain steady frequency on the generator output, you don't need a soft starter or vfd.    

RE: Generator sizing doubts

Not always. I experienced a group of soft starts at a remote sawmill. The soft starts had trouble on the grid as we had unbalanced voltages and a displaced neutral on the grid. The soft starts would trip out on phase angle error from the neutral issues.
They started almost well on the generators.
By that I mean that we had two generators in parallel and needed both generators to start the large motors even with the soft starts.
One generator had a hydraulic governor and the other had an electronic governor.
When the motors started tripping out on phase angle error we would start the generators but if we tried to start immediately  the hydraulic governor could not respond fast enough to prevent the electronic governed machine from hogging the load and tripping the main breaker on the gen-set. After warming up for 15 or 20 minutes the oil would be thin enough that gen-sets would share the load well enough to avoid tripping the breaker.
Al;though we had phase angle issues on the grid we never had frequency issues when running on the gen-sets.
A VFD with a passive front end is just rectifying the AC.  It should be even more forgiving of frequency excursions than a soft start.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Generator sizing doubts

(OP)
Given the application of two generators in parallel, how do you prevent backfeeding?  And does it matter if there is backfeeding in small amounts?  Suppose the AVRs were not perfectly similar, and gen 1 output 482V and gen 2 output 479V.  At a glance I'd say that some small percentage of gen 2 would become a load to gen 1.  This imbalance seems inevitable.  Does it cause any damage?

RE: Generator sizing doubts

People write pages and chapters on these subjects but quickly;
Generators in parallel.
1> The load (KW) that each generator will accept depends on the throttle setting of the engine or prime mover. This may cause a slight interaction with the frequency.
The Classic control scheme is to run the governors in droop mode. At no-load the generators run 3% fast and drop to nominal speed/frequency at full load.
2> The VAR sharing depends on the voltage setting. There is a scheme called cross current compensation or the quadrature circuit which biases the AVRs to compensate for small inequalities in AVR settings. In practice, with a normal load, the set in your example that has the higher voltage setting will take a larger share of the VAR demands. If your example voltages are stand-alone voltages, the cross currrent compensation will work to reduce the difference when the sets are in parallel.
3> Do the imbalances cause damage? Usually small imbalances cause one set to take more than its share of the load. Governor issues cause KW sharing issues. Voltage setting issues cause VAR sharing issues.
4> One set may take more than its share of the KW load while the other set takes more than its share of the VAR demand. There is very little interaction if any between speed settings and voltage settings on paralleled sets.
BUT
If the sets are heavily loaded, sharing issues may cause one set to be overloaded. This may be a current overload (with heating issues), a mechanical overload with prime mover issues or both.
Governor issues may cause mechanical overloading which may lead to overcurrent issues.
Voltage setting issues generally lead to overcurrent and heating issues.
And, a severe unbalance under light loading may cause the unloaded set to "Motor". This does not generally cause immediate damage but if the condition persists may lead to wet stacking or slobbering. This is a carryover of lube oil into the exhaust system. Some piston rings particularly when old, do not seal well without the force of combustion. Also, any lube that passes the rings will eventually be pushed out into the exhaust system instead of being burned with the fuel in the combustion chamber. When the engine is again put under load, the wasted lube oil may be ignited in the exhaust system, it may be expelled, (messy) burning lube oil may be expelled from the exhaust or an explosion inside the exhaust system may result.
This used to happen when the operator tried to stop a generator but forgot to open the circuit breaker first. A common design feature is a reverse power relay/trip to avoid this event. The last time I saw this happen I replaced a faulty reverse power relay.
Another possible issue may be damage to the fuel injection system. Some injection pumps depend on the flow of fuel for lubrication and extended operation with no fuel flow may cause damage. While most diesel engines will run on jet fuel, it will also lead to damage to some injector pumps due to insufficient lubricity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Generator sizing doubts

A soft start that I commonly use has a rated frequency range of +-20% of 60Hz.  I'm not sure if this is typical.  Frequency dip on a generator should be 10% at worst.

RE: Generator sizing doubts

Beware of where you control power (contactor) comes from - you may have enough generator power but the volt drop can drop out the contactor (around 80%) if it is powered from the one phase and neutral, often the case for larger contactors.

RE: Generator sizing doubts

X49; Your question regarding PMG equipped generators;
These hold the voltage up very well as long as the frequency holds up. When you are starting large motors on a gen set you often have a frequency drop. Depending on the size ratio between the generator and motor this may be until the governor opens up enough to pick up the load or it may be sustained longer.
If the generator is lightly loaded and turbo charged, you may have a frequency drop while the turbo spools up.
When the frequency drops more than 3 Hz the UFRO function of the AVR will drop the voltage regardless of the voltage available from the PMG exciter supply.
Note: The Permanent Magnet Generator supplies power to the AVR to avoid voltage collapse on faults. The main generator is not changed and may be excited with a conventional AVR in an emergency while waiting for parts should the PMG add-on fail. There may be protection and block loading issues but the set will generate and supply most loads.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

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